Not applicable.
Not applicable.
The present invention relates to optical elements for creating polarized light in general, and to optical elements for creating circularly polarized light in particular.
Circularly polarized light has many uses, particularly where it is desirable to gain the benefits of linearly polarized light, without the variability with viewing angles. Circularly polarized light may be used in ellipsometry, to study the properties of thin films. Moreover, according to my earlier inventions disclosed in U.S. Pat. Nos. 6,219,139 and 6,055,053, the disclosures of which are incorporated herein by reference, circularly polarized light can be used to detect shear stress magnitude and direction, in thin coatings on parts undergoing stress or in glass sheets and windows.
Circularly polarized light is generated by creating linearly polarized light with a linear polarizer and passing the resulting linearly polarized light through a xc2xc wave plate perpendicular to the principal optical axes and rotated with respect to the plane of polarization by forty-five degrees. A xc2xc wave plate is an optical element which causes the light vibrating in a first optical plane to experience a xc2xc wavelength delay as compared to light passing through a second optical plane perpendicular to the first optical plane.
A minimum delay, or fast, axis is defined for a birefringent material along which light passes through the material with minimum delay. A maximum delay, or slow, axis is defined along an axis perpendicular to the axis of minimum delay, along which light is subjected to the maximum retarding effect.
Index of refraction is defined as the ratio between the speed of light in a vacuum and the speed of light in a material. In certain materials the index of refraction, that is the speed with which light travels through the material, depends on the orientation of the vibrational plane of the light with respect to orientation of the atoms making up the material. For example, when ordinary light passes through a calcite crystal along a particular crystallographic axis the strong birefringence exhibited by the crystal resolves the light into two polarized images. These images are spatially separated because the light which is experiencing the higher index of refraction experiences greater refraction.
Circularly polarized light is plane polarized light where the plane of polarization rotates about a line parallel to the direction of propagation of the light. Circularly polarized light can also be thought of as polarized light where the orientation of the plane of polarization has a probability function evenly distributed about the direction of propagation of the light. Thus when circularly polarized light is viewed through a plane polarizing analyzer only a component of the circularly polarized light will pass through the analyzer producing a neutral grey to the observer.
When plane polarized light encounterers a second plane polarizing filter, the intensity of light which passes through the second filter is proportional to the cosine of the angle between the axis of the polarized light and the axis of the polarizing filter. When circularly polarized light encounterers a plane polarizing filter, the amount of light which passes through the polarizing filter is independent of the orientation of the filter and can be viewed as the summation of all possible orientations times the cosine between each possible orientation and the axis of the polarizing filter. This results in the same light intensity as if the circularly polarized light were plane polarized and oriented forty-five degrees from the axis of the polarizing filter.
However, although a plane polarizer produces plane polarized light with respect to every frequency or color of light, a xc2xc wave plate only produces circular polarized light with respect to a single wavelength or color of light, because the wave plate causes a delay along the slow axis which is the same for every wavelength but which is equal to a quarter of a wavelength only for a single selected wavelength or color of light. Where the light has a longer wavelength than the selected wavelength, the light produced is elliptically polarized with the major axis of the light parallel to the plane of the polarized light. Where the light has a shorter wavelength than the selected wavelength, the light produced is elliptically polarized with a major axis of the line perpendicular to the plane of the polarized light.
Achromatic circular polarizers are known, for example a Fresnel rhomb, which is a specially shaped rhomb of glass that totally internally reflects a beam of light twice. The Fresnel rhomb is nearly achromatic, but has drawbacks as regards to bulk and cost. A combination of plastic films such as described in U.S. Pat. No. 2,441,049 can be used to produce achromatic circularly polarized light, however such a combination may not be stable over time and the cellulose nitrate used in the combination may constitute a fire hazard. Stacked multiple crystalline plate systems are described, for example by Pancharatnam, in Proc. Indian Acad. Sci A 41,130 and 137 (1955), however, these achromatic circular polarizes are expensive and of limited aperture. What is needed is a low cost method of fabricating an achromatic circular polarizer with a large aperture.
The achromatic circular polarizer of this invention comprises optical elements spaced along a defined optical axis, beginning with a plane polarizer, defining a plane of polarization, followed by a xc2xd wavelength plate oriented with the principal optical axes at forty-five degrees to the defined plane of polarization, followed by a plurality of glass-air interfaces, formed by a plurality of glass plates, which are angled between about 45 and 55 degrees with respect to the optical axis. Finally a xc2xc wave plate is positioned after the plurality of glass plates, perpendicular to the optical axis and oriented with the principal optical axes oriented minus forty-five degrees to the defined plane of polarization, and thus opposite in orientation to the xc2xd wavelength plate. The wavelength of the xc2xc and xc2xd wave plates, is preferably selected for green light. As light of a wavelength which differs from green light passes through the xc2xd wave plate it acquires an error which is twice the magnitude, and opposite in sign, of the error which the same wavelength will acquire when passing through the xc2xc wave plate. Polarized light which vibrates parallel to the plane 48 as shown in FIG. 9 is attenuated by the glass plates 40 of the partial polarizer 38. The number of and angle of the glass plates 40 is selected to reduce the error in half. The light thus corrected passes through the xc2xc wave plate, where the polarized light at the green wavelength is converted to circularly polarized light, and the light at other wavelength, having been precondition with an error which is opposite in sign (direction) to the error induced by the xc2xc wave is converted to circularly polarized light. This optical train thus forms an achromatic circular polarizer.
It is an object of the present invention to provide an achromatic circular polarizer of low cost.
It is a further object of the present invention to provide an achromatic circular polarizer which can easily be constructed with a large viewing aperture.
It is another object of the present invention to provide an achromatic circular polarizer which can be readily adjusted.
Further objects, features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.